An antilock brake system of this kind is well known and is described, by way of example, in Japanese Patent Application Laid-Open No. 62-134361. It is described in this laid-open publication that a motor is rotated continuously from the start of a slow increase in pressure the first time the increase is performed to the end of antiskid control in order to avoid a lag in pressure increase when fluid brake fluid pressure is slowly increased during antilock control, and to make it possible to drive two pumps by the single motor.
In a case where the motor is thus rotated continuously from the start of the first slow increase in pressure to the end of antiskid control, the brake fluid that has been fed under pressure from the pump to the wheel brake flows out to the low back-pressure reservoir through the control valve operating in the pressure-reducing condition during a decrease in pressure from the second time onward. As a result, brake fluid pressure (fluid pressure within the wheel brake) during the pressure decrease cannot be reduced to the fluid pressure of the low back-pressure reservoir. In other words, the amount of brake fluid that flows out from the wheel brake to the low back-pressure reservoir via the control valve varies in dependence upon the pressure differential between the brake fluid pressure and the fluid pressure in the low back-pressure reservoir. The amount of brake fluid outflow decreases as the brake fluid pressure declines. When the amount of brake fluid that flows from the wheel brake to the low back-pressure reservoir via the control valve has decreased to an amount of flow the same as that of the brake fluid fed from the pump, the brake fluid pressure stops falling.
The amount of flow of brake fluid fed under pressure from the pump to the wheel brake generally is set so as to obtain an appropriate slow pressure-increase gradient for a road surface on which antilock control is performed with a high degree of frequency (namely a road surface for which the coefficient of friction is higher than that of an iced road surface).
In accordance with the prior art, therefore, when antilock control is performed with the vehicle running on an iced road surface, there is the fear that locking of the wheels will be unavoidable because it will not be possible to reduce the brake fluid pressure sufficiently. The object of the invention of this application is to arrange it so that an appropriate slow pressure-increase gradient can be obtained for a road surface having a high coefficient of friction, and so that brake fluid pressure can be reduced sufficiently for a road surface having a low coefficient of friction, such as the coefficient of friction of an iced road surface, i.e., so that the brake fluid pressure can be reduced to the fluid pressure of the reservoir.